In general, a thermally sensitive recording medium having a thermally sensitive recording layer that contains as main components a colorless or pale colored dye precursor and a color developing agent reacting with the dye precursor under heating to develop color is put into practical use extensively. A thermal printer incorporated with a thermal head or the like is used for recording on the thermally sensitive recording medium. Such a thermally sensitive recording method has features in that the method produces no noise during recording, requires no developing or fixing, is maintenance-free, employs relatively inexpensive and compact instruments, and provides very clear color development, compared with other conventional recording methods in practical use. Thus, the thermally sensitive recording method is used extensively in a facsimile or computer field, for various measuring instruments and labels, and the like with the development of the information industry.
Required qualities for the thermally sensitive recording medium have increased to a higher level with the progress in diversification of recording devices and in the production of high performance recording devices. Regarding color development sensitivity, a high density image having a clear color development has been demanded even with a very small thermal energy with the progress in miniaturization of devices and speeding up of recording. In order to satisfy the demand, Patent Document 1 describes a method of enhancing the color development sensitivity by using a novel color developing agent, for example.
Patent Document 1 JP-A 2002-301873
Storage stability of a color image to a natural environment such as heat, water, humidity, or light, to body fat from handling of the color image by hand, to oil, a plasticizer, a solvent, or the like, and favorable background color are demanded for the thermally sensitive recording medium with an expansion of its application. Of the required qualities, it is particularly difficult to provide the thermally sensitive recording medium with an image stability to light. Moreover, a high color development sensitivity and a high thermal resistance are opposite characteristics, and it is very difficult to attain both characteristics.
Various improvements have been made on the resistance of a color image, and color images that withstand use under long-term storage or harsh storage conditions (stability with time, temperature and humidity resistance, water resistance, chemical resistance, or friction resistance, for example) have been put into practical use. As a result, the use of the recording medium is not simply limited to the field of information recording. A colored image having an improved resistance allows information storage, and the recording medium itself has also been used as a note.
When the thermally sensitive recording medium as described above is used as a note, the recording medium is often brought into contact with oil of a human hand, a plasticizer for synthetic leather used in wallets, and the like, to thereby cause a disadvantage in that a color image fades significantly. Further, a color image must be clear, even after long-term storage, and favorable thermal response and storage stability of the color image are desired. However, it is particularly difficult to provide the conventional thermally sensitive recording medium with an image stability to light. No thermally sensitive recording medium with a sufficient quality and having a good balance among color development sensitivity, image storage stability and the like has been obtained.
When a thermally sensitive recording medium is applied to uses as notes such as tickets, voting cards, bonds, and receipts, an input of much information is desired and easy confirmation of authenticity of issued notes is demanded. In a case of a lottery ticket for a lottery or the like, variable information such as a serial number is printed on the ticket in advance during a ticket processing step for preventing falsification or alteration thereof. An inkjet method has recently spread as means of printing such information, and a thermally sensitive recording medium having inkjet recordability has been strongly desired. Patent Documents 2 and 3 each describe a thermally sensitive recording medium having inkjet recordability on a back surface thereof.
Patent Document 2 JP-A 2000-203163
Patent Document 3 JP-A 2000-318319
However, those are essentially thermally sensitive recording media and may be exposed to rain outside or used under a high humidity and are apt to cause problems such as a bleeding phenomenon of an ink image (feathering), a small ink optical density, and flowing out of the ink when the image gets wet with water. Thus, water resistance for withstanding the problems is demanded for the thermally sensitive recording medium. Recording media used in the inkjet recording method are roughly classified into a plain paper type having a texture similar to that of so-called good quality paper/electrophotographic paper (PPC paper), and a coated paper type having an inkjet recording layer (ink receiving layer). In particular, an inkjet recording medium of a plain paper type has a fiber on a paper surface exposed, so feathering is apt to occur. Regarding inkjet recordability, high color developing ability, ink-absorbing ability, color reproducibility, and the like are required, and full color recording requires higher ink-absorbing ability, in particular.
Meanwhile, the inkjet recording medium may have offset printability on a surface and inkjet recordability on a back surface for uses as postcards or the like. For example, Patent Document 4 describes a multi-layered sheet of three or more layers having an oil-absorbing filler with an oil absorption of 80 ml/100 g or more on a back layer serving as an inkjet recording surface.
Patent Document 4 JP-A 9-143900